Method for evaporation and/or condensation in a heat exchanger

ABSTRACT

The invention concerns a method for evaporation and/or condensation of at least one fluid in a heat exchanger consisting of a stack of at least one tube ( 3 ) and of at least one folded corrugation ( 17 ), the corrugation and the tube being preferably brazed together, wherein a fluid flows inside at least one tube, and another fluid flows around the corrugation ( 17 ). The invention also concerns an installation for separating a mixture of fluids by cryogenic distillation, comprising a heat exchanger operating in accordance with such a method.

The present invention relates to a method for vaporization and optionally condensation of a fluid in a heat exchanger and to an installation for separating a mixture of fluids by cryogenic distillation, which includes at least one heat exchanger operating according to such a method. In particular, it relates to a method for vaporization and optionally condensation of air gases in an installation for separating air gases by cryogenic distillation.

Air gas separation units have used brazed aluminum plate heat exchangers for a very long time for reboiler/condenser functions of the distillation columns, especially the reboiler/condenser of the double column with nitrogen condensing and oxygen vaporizing.

Two operating principles for these reboiler/condensers have been proposed:

-   -   in thermosiphon mode: this is the oldest solution, with two         variants:     -   1) the body of the vaporizer may be placed vertically and         completely (or partly) immersed in a liquid oxygen bath. This         liquid oxygen enters at the bottom of the vaporizer, is warmed         up to its bubble point and then partly vaporized. The         recirculation ratio (flow of excess liquid at the outlet to the         vaporized flow) is very high, possibly ranging from 5 to 100.         The dimensions of the bodies may be around 1220 mm in width×1200         mm in stacking direction×2000 mm in length. One of the drawbacks         is that even though the minimum temperature difference in the         exchanger may be low (0.3-0.4° C.), the apparent difference (the         difference between the temperature of the nitrogen entering the         reboiler/condenser and the temperature of the vaporized oxygen)         remains high (1.2-1.6° C.) because of the hydrostatic head of         liquid, which increases the pressure of said liquid at the         bottom of the exchanger and therefore its vaporization         temperature. It would be conceivable to reduce the hydrostatic         head by not completely immersing the vaporizer, but then the         recirculation ratio would be reduced. However, it is dangerous         to operate with zero (or low) recirculation since by entirely         vaporizing the liquid oxygen (generally or locally), the         solubility limit of certain heavy constituents (for example         hydrocarbons) is reached in the last drop of liquid to be         vaporized. These heavy constituents run the risk of being         deposited in the vaporizer and causing an explosion therein and         combustion of the aluminum; and     -   2) the body of the vaporizer is placed horizontally and         completely (or partly) immersed in a liquid oxygen bath. The         operation is identical to that of the vertically installed         vaporizer. By reducing the hydrostatic head, the operation on         the oxygen side is improved. However, the design of the nitrogen         passages is not obvious. Either the length of the exchanger body         (which is cubic) is limited and the same plane of flow is used         as for the vertical bodies, or the same body length (2000 mm or         more) is retained but it is then necessary to make the nitrogen         flow horizontally (otherwise the distributors would assume the         entire height of the body) with cross-flow, thereby resulting in         heating asymmetry, which may degrade the performance, and also         defects in discharging the liquid nitrogen (gravity driving it         toward the bottom of the passages), which also locally degrades         the exchange coefficient; and     -   in film mode: a more recent solution, described in particular in         U.S. Pat. No. 4,599,097.

The liquid is distributed over vertical plates. The hydrostatic pressure therefore no longer adversely affects the exchange and small temperature differences (of less than 1° C.) may be obtained. However, if it is desired to provide excess liquid at the outlet of the exchanger, to avoid dry vaporization and deposition of heavy constituents, a pump is needed. Put another way, the operation is potentially dangerous for the same reasons as for reboilers/condensers.

EP-A-1 008 826 proposes a falling-film evaporator in which the exchanger comprises passages defined by parallel plates. The liquid vaporization passages contain auxiliary passages that have only curved surfaces, for example cylindrical tubes.

Moreover, two design types exist: reboiler/condenser inside a column or a shell, or an external vaporizer, for example described in U.S. Pat. No. 5,333,683.

One object of the invention is to provide a condensation and/or vaporization method using a heat exchanger that alleviates the drawbacks of the prior art and more generally an alternative heat exchange method to that carried out in a brazed aluminum plate exchanger, derived from the technology currently used in automobile radiators.

For this purpose, one subject of the invention is a method for the vaporization and/or condensation of at least one fluid in a heat exchanger consisting of a stack of at least one tube and of at least one corrugated fin, the fin and the tube being preferably brazed to each other and in which heat exchanger a first fluid, optionally to be condensed, flows inside at least one tube and a second fluid, optionally to be vaporized, flows around the fin, in which a) the first fluid condenses and the second fluid vaporizes or b) the first fluid vaporizes and the second fluid condenses.

The method according to the invention may furthermore comprise one or more of the following features:

-   -   the corrugation of the fins is approximately parallel to the         axis of the tubes;     -   the tubes and the fins are made of pure or alloyed aluminum;     -   the tubes and the fins are made of a copper-based alloy;     -   the tubes and the fins are made of an iron-based alloy;     -   the tubes are oblong and/or flattened;     -   part of the exchange area is inside the tubes;     -   the tube has parallel channels on the inside, comprising two         parallel flat walls and internal walls which are connected to         the two flat walls and define the parallel channels;     -   the exchange area inside the tubes is obtained by folding, by         extrusion or by preferably brazed inserts, for example fins; and     -   the fins are perforated, straight, serrated and/or louvered.

Alternatively, it would be conceivable for the vaporization to take place in the tubes, and the subject of the invention would then be a method for vaporization and optionally condensation of at least one fluid in a heat exchanger consisting of a stack of at least one tube and at least one corrugated fin, the fin and the tube preferably being brazed to each other, and in which a fluid to be vaporized flows inside at least one tube and another fluid, optionally to be vaporized, flows in channels generated by fins.

The invention aims more particularly to provide a method for vaporization of at least one liquid derived from air and optionally for condensation of at least one gas derived from air, or which is air, as described above.

The aim of the invention is also to provide a method for vaporization of at least one liquid having methane and/or carbon monoxide and/or hydrogen as main component and optionally for condensation of at least one gas having methane and/or carbon monoxide and/or hydrogen as main component, as described above.

Finally, the object of the invention is to provide an installation for separating a mixture of fluids by cryogenic distillation in at least one column having at least one heat exchanger operating according to a heat exchange method in a heat exchanger consisting of a stack of at least one tube and of at least one corrugated fin, the fin and the tube preferably being brazed to each other, and in which a fluid flows inside at least one tube and another fluid flows around the fin, one is heated while the other is cooled.

Preferably, at least one of the heat exchangers of such an installation is one of the types below:

-   -   i) a reboiler/condenser for vaporization of a liquid by heat         exchange with a gas, which condenses inside or outside a         distillation column; or     -   ii) a subcooler; or     -   iii) a regeneration gas heater for a purification unit used for         purifying the mixture to be distilled; or     -   iv) a dephlegmator; or     -   v) a heat exchanger having passages for cooling the mixture to         be distilled at a cryogenic temperature; or     -   vi) an exchanger for cooling an interstage of a compressor for         compressing the mixture to be distilled or a product of the         distillation.

There are many advantages of such a solution:

-   -   1) the manufacture of the exchangers may benefit from the         infrastructures available for manufacturing automobile         radiators, allowing a very substantial reduction in the cost and         lead times in manufacturing these exchangers. At the present         time, it takes several months to design and manufacture brazed         aluminum bodies. Using the technology of automobile radiators,         once standard sizes have been defined, their manufacture will         require only a few days;     -   2) this technology will also allow other alternatives as regards         materials. Admittedly, aluminum will remain one option, but it         will be possible to produce copper/bronze or stainless steel         exchangers. At the same time, by obviating the risk of aluminum         catching fire (for example by using copper-based alloys), the         use of reboilers and falling-film condensers is made safer;     -   3) using the conventional technology of brazed aluminum plate         exchangers, when it is desired to place them in the circular         shell of a distillation column, the useful cross section         (exchange area) occupies only about 50% of the cross section of         the column. Using this “radiator” technology, there is no longer         a need to place the boxes and headers at the same height as the         exchange bodies. They may be advantageously placed above and         below the exchangers; and     -   4) by no longer brazing large bodies and using copper-based         alloys, there is no longer the risk of the fins collapsing or         the risk of aluminum catching fire, since the material is much         stronger and does not release energy in the event of combustion,         and the thickness of the fins on the vaporization side may         therefore be considerably reduced (down to 50 μm).

However, even though the automobile radiator technology may be used, certain adaptations are necessary in order to make it even more beneficial for use in a reboiler/condenser:

-   -   1) replacement of the cross-flow with countercurrent flow in the         case of vaporization in thermosiphon mode: the headers are         placed vertically in an automobile radiator. In a thermosiphon         reboiler/condenser, in which the direction of the fluid         undergoing condensation in the tubes is countercurrent to the         direction of the vaporizing fluid in the channels generated by         the fins, owing to condensation in the tubes, the gas header         wilt be placed at the top and the liquid header at the bottom,         so as to make gravity flow of the liquid easier;     -   2) replacement of the cross-flow with cocurrent flow in the case         of falling-film vaporization: in this case, the direction of the         fluid undergoing condensation in the tubes is cocurrent with the         direction of the fluid vaporizing in the channels generated by         the fins, and a device for feeding the liquid to be vaporized         must be added at the top of the fins;     -   3) in a radiator, water is often cooled in the tubes against the         air in the fins. Since the exchange coefficient on the air side         is appreciably lower than on the water side (by a factor of at         least 10), the exchange area is increased on the air side thanks         to the fins. In the case of vaporization/condensation, the         exchange coefficients on the two sides are of the same order of         magnitude. It is therefore beneficial to have similar exchange         areas, hence the solution consisting in having additional         surface inside the tubes: inserts, fins (brazed or not),         corrugations, extrusion, etc. (U.S. Pat. No. 6,241,012); and     -   4) such a structure could also operate as a dephlegmator.

Finally, the use of exchangers derived from automobile radiator technology in gas separation by cryogenic distillation is not limited to reboiler/condensers, which vaporize a fluid by heat exchange with another fluid, which condenses, but may also be used for:

-   -   regeneration gas heaters for purification units;     -   subcoolers;     -   principal exchangers, especially for vaporizing under pressure         gaseous oxygen in tubes made of a copper-based alloy with         respect to air, which condenses; and     -   compressor interstage cooling exchangers.

Particular embodiments of the invention will now be described with reference to the appended drawings, in which:

FIG. 1 is a front elevation view of a heat exchanger of a first type according to the invention;

FIG. 2 is a schematic perspective view of a heat exchanger for implementing the method according to the invention;

FIG. 3 is a perspective view, on a larger scale and along the same direction, of part of the exchanger shown in FIG. 1 according to a first embodiment;

FIG. 4 is a perspective view showing a tube portion of the exchanger of FIG. 2 according to a second embodiment;

FIG. 5 is a sectional view, in a vertical plane, of a stack of tubes and fins of a heat exchanger according to a third embodiment; and

FIGS. 6 and 7 are schematic views, of the top and front respectively, of a reboiler/condenser of an installation according to the invention, which contains exchangers of a second type, similar to the first shown in FIGS. 1 and 2.

FIG. 1 shows schematically a heat exchanger 1 having a structure similar to that of the exchangers used in motor vehicle cooling circuits.

For convenience in the following description, FIGS. 1 to 5 will be oriented with respect to the orthogonal reference frame X, Y, Z, in which:

-   -   the X and Z axes define the vertical plane of FIG. 1 and the         principal directions along which the exchanger 1 lies, the X         axis being supposed to be horizontal and the Z axis supposed to         be vertical; and     -   the Y axis is the transverse horizontal axis.

The exchanger shown in FIG. 1 essentially comprises, on the one hand, a stack of elongate tubes 3 spaced apart and mutually parallel, which extend horizontally along the X axis and, on the other hand, corrugated oblong fins (not visible in FIG. 1) placed in the gaps between two consecutive tubes 3.

The tubes 3 are connected at one of their ends to a distribution column 5 and at their other end to a collecting column 7. The two columns 5, 7 are formed from vertical tubular pipes in fluid communication with each of the tubes 3. Preferably, the tubes 3 are brazed to the columns 5, 7, said columns being formed beforehand so as to allow the tubes 3 to be fitted into them. These columns are not necessarily of cylindrical shape. Each may be a tubular plate recessed so as to allow the tubes to be fitted into it, onto which plate the tubes will preferably have been brazed and to which a box, typically of semicylindrical shape, will be attached, for example by welding after the brazing operation.

The distribution column 5 is equipped in an upper part with a fluid inlet coupler 9 allowing the exchanger 1 to be supplied with a first fluid.

The collecting column 7 is correspondingly provided, in a lower part, with an outlet coupler 11 for evacuating the first fluid from the exchanger 1.

The couplers 9, 11 are shown schematically in FIG. 2.

The exchanger shown in FIG. 2, like that in FIG. 1, essentially comprises, on the one hand, a stack of elongate tubes 3, spaced apart and mutually parallel, and, on the other hand, corrugated oblong fins (not visible in FIG. 2) placed in the gaps lying between two consecutive tubes 3. Now, in the case of the invention, the elongate tubes 3 extend vertically along the Z axis and the corrugation or folding direction of the fins 17 in FIG. 2 is parallel to the longitudinal axis of the tubes 3, that is to say parallel to the Y axis.

The tubes 3 are connected at their upper end to a distribution column 5 and at their other end to a collecting column 7. The two columns 5, 7 are formed from vertical tubular pipes placed horizontally and in fluid communication with each of the tubes 3. Preferably, the tubes 3 are brazed to the columns 5, 7, said columns being formed beforehand so as to allow the tubes 3 to be fitted into them. These columns are not necessarily of cylindrical shape. Each may be a tubular plate recessed so as to allow the tubes to be fitted into it, onto which plate the tubes will preferably have been brazed and to which a box, typically of semicylindrical shape, will be attached, for example by welding after the brazing operation.

The distribution column 7 is equipped on the left with an inlet coupler 11, allowing the exchanger 1 to be supplied with a first fluid in gaseous form. The coupler extends perpendicularly to the axis of the distribution column and to the axis of the tubes. This coupler may nevertheless extend along another direction, for example along the Z axis or possibly the Y axis.

The collecting column 5 is correspondingly provided, in a lower part, with an outlet coupler 9 for evacuating the first fluid from the exchanger 1. The coupler extends perpendicularly to the axis of the distribution column and to the axis of the tubes. However, this coupler could extend in another direction, for example along the Z axis or possibly the Y axis.

The couplers 9, 11 have been shown schematically in FIG. 2.

In the case of vaporization in thermosiphon mode, a fluid to be vaporized (a second fluid) flows over the fins 17, in an ascending vertical direction (i.e. the fluid to be vaporized is made to flow in the channels generated by the corrugations), and a fluid at higher temperature (first fluid) is made to flow inside the tubes 3 along a descending vertical direction.

In the case of falling-film vaporization, a fluid optionally to be condensed flows over the fins 17 in a descending vertical direction (i.e. the fluid optionally to be condensed is made to flow in the channels generated by the fins) and a lower temperature fluid to be vaporized is made to flow inside the tubes 3 along a descending vertical direction.

FIG. 3 shows a portion of part of the exchanger 1 of FIG. 1, consisting of two consecutive tubes 3 and a corrugated fin 17 provided between these two tubes.

As may be seen in this figure, the tubes 3 have a running section, in the XY vertical plane, of transversely elongate shape along the X axis, so that they each have two approximately plane and parallel opposed faces. In other words, the tubes 3 have an oblong cross section on the transverse axis X that is of flattened shape.

The fin 17 is corrugated along a corrugation or folding direction Y perpendicular to the longitudinal axis of the tubes 3. The fin 17 is fixed to the tubes 3, preferably by brazing, at its peaks 19. This brazing operation may be concomitant with the brazing of the tubes 3 to the columns 5, 7.

The fins 17 may be of any suitable type, for example one of the following types commonly used in plate heat exchangers, namely: perforated fins, straight fins, serrated (partially offset) fins, herringbone (zig-zag) fins and louvered fins.

The fins 17 may have, in cross section in the YZ plane, a sinusoidal, rectangular or triangular shape, or may have any other suitable type of geometric pattern.

The hydraulic diameter of the channels formed by the fins 17 is typically between 100 μm and 10 mm.

These fins may be made of solid sheet metal, perforated sheet metal, sintered metal or any other metal structure (foam, etc.).

The tubes 3 and the fins 17 may be made of pure or alloyed aluminum.

As a variant, the tubes 3 and the fins 17 may be made of a copper-based alloy.

As another variant, the tubes 3 and the fins 17 may be made of an iron-based alloy.

The exchanger 1 in FIG. 2 has fins 17 no longer oriented along the X axis but along the Z axis.

In the example shown in FIG. 4, the internal volume bounded by each tube is divided longitudinally into two. To do this, the tube 103 has its upper face cut along a longitudinal mid-line, the two edges 21, 22 separated by this line being turned down toward the interior of the tube and welded to the lower wall. The strips thus turned down are contiguous and form a double wall separating the two longitudinal compartments 103A, 103B thus defined. These compartments are called channels.

The edges 21, 22 may for example be welded to the lower wall by laser welding.

A heat exchanger made up from tubes of this type is better able to withstand the pressure of the fluid flowing in the channels 103A, 103B since they are smaller than the tube 3. Such a design may be used to generate a number of channels greater than 2.

Incidentally, a heat exchanger made up from tubes of this type is capable of operating with three different fluids, one flowing over the fins, another flowing in the channel 103A and the third flowing in the other channel 103B.

Thus, it is possible to make two different fluids flow in the two channels 103A, 103B, between which fluids a heat exchange takes place so that part of the exchange area, obtained by folding the edges 21, 22, is inside the tube 103.

FIG. 5 illustrates another embodiment of a stack of tubes and fins suitable for implementing the method according to the invention.

In this stack, the tubes 203 are again tubes of transversely elongate cross section having plane and parallel opposed faces. However, their internal volume is divided into a plurality of parallel longitudinal channels 203A separated by mutually parallel plane walls 23, which here are vertical. The hydraulic diameter of these channels is typically between 100 μm and 10 mm.

The walls 23 can be made as a single entity with the external walls of the tube 203, for example by extrusion, or else they may consist of inserts, preferably brazed inserts. These inserts may be very similar to the fins 17.

In the example shown, each layer of tubes consists of two adjacent parallel tubes lying in the same plane.

Thus, part of the exchange area lies inside the tubes 203.

Optionally, a wall 204 surrounds the tubes 203 and the fins 17 so as to seal the exchanger from its environment. This wall may be brazed to the tubes 203 or simply enclosed around the tubes 203.

In the embodiment shown in FIG. 5, and unlike the embodiment shown in FIG. 3, the fluid to be vaporized that flows over the fins 17, flows parallel to the fluids to be condensed that flow in the tubes 203. Preferably, the fluids to be condensed on the one hand and the fluid to be vaporized on the other will flow in opposite directions.

FIGS. 6 and 7 show schematically part of the installation according to the invention, which comprises, inside a shell 31 of cylindrical general shape, a series of exchangers 301 a-301 n of the same type, similar to that shown in FIGS. 1 and 2.

In these figures, the orthogonal reference frame X₀, Y₀, Z₀ shown is defined as follows:

-   -   the Z₀axis is the upwardly oriented vertical axis;     -   the X₀ axis is the horizontal axis defining, with the Z₀axis,         the principal planes in which the exchangers 301 lie; and     -   the Y₀axis is the horizontal axis orthogonal to the X₀axis.

The exchangers 301 a-301 n are all parallel to one another and centered with respect to a diametral plane of the cylindrical shell 31, as may be seen in FIG. 6. The length of each exchanger 301 a-n is adjusted to the length, along the X₀ axis, of the vertical cross section of the shell 31 on the Y₀ axis. Thus, the length along the X₀ axis of the exchangers 301 a-301 n increases toward the central axis Z₀ of the cylindrical shell 31.

Unlike the orientation of the exchanger 1 shown in FIG. 1, the orientation of the exchangers 301 a-n is such that the distribution columns 305 a-n extend horizontally from the upper side of the shell 31, whereas the collecting columms 307 a-n extend from the lower side of the shell 31, again horizontally. Thus, the stacks of tubes 303 extend between the columns 305 a-n, 307 a-n, along the vertical axis Z.

Placed above the distribution columns 305 a-n there is a gas header 41 designed for supplying the group of exchangers 301 a-n with gas.

The installation shown also includes a liquid header 43 placed beneath the collecting columns 307 a-n and designed to collect the liquid phase coming from the group of exchangers 301 a-n.

The fins stop before the columns 307 a-n and 305 a-n so as to allow the fluid to enter and leave. The external wall shown in FIG. 5 (204) stops substantially at the same levels as the fins, again to allow the fluids to enter and leave.

The vaporization/condensation method that has been described above and the installation described with reference to FIGS. 6 and 7 applies to the vaporization of at least one liquid derived from air or a liquid which is liquefied air, and to the condensation of at least one gas derived from air, this gas possibly also being air itself.

The method and the installation also apply to the vaporization of at least one liquid having methane and/or carbon monoxide and/or hydrogen as principal component and to the condensation of at least one gas having methane and/or carbon monoxide and/or hydrogen as principal component.

Such a method may apply to many types of installations for separating mixtures of fluids, operating by cryogenic distillation, in at least one column having one or more heat exchangers such as those described above.

The installation may in particular be:

-   -   a reboiler/condenser for vaporization of a liquid by heat         exchange with a gas, which condenses inside or outside a         distillation column; or     -   a subcooler; or     -   a regeneration gas heater for a purification unit used for         purifying the mixture to be distilled; or     -   a dephlegmator; or     -   a heat exchanger having passages for cooling the mixture to be         distilled at a cryogenic temperature; or     -   an exchanger for cooling an interstage of a compressor for         compressing the mixture to be distilled or a product of the         distillation. 

1-17. (canceled)
 18. A method for the vaporization and/or condensation of at least one fluid in a heat exchanger consisting of a stack of at least one tube and of at least one corrugated fin, the fin and the tube being preferably brazed to each other and in which heat exchanger a first fluid, optionally to be condensed, flows inside at least one tube and a second fluid, optionally to be vaporized, flows around the fin, in which a) the first fluid condenses and the second fluid vaporizes or b) the first fluid vaporizes and the second fluid condenses.
 19. The method of claim 18, in which the corrugation of the fins is approximately parallel to the axis (X or Z in the case of FIG. 2; Z₀) of the tubes.
 20. The method of claim 18, wherein the tubes and the fins are made of pure or alloyed aluminum.
 21. The method of claim 18, wherein the tubes and the fins are made of a copper-based alloy.
 22. The method of claim 18, wherein the tubes and the fins are made of an iron-based alloy.
 23. The method of claim 18, wherein the tubes are oblong and/or flattened.
 24. The method of claim 18, wherein part of the exchange area is inside the tubes.
 25. The method of claim 18, wherein the exchange area inside the tubes is obtained by folding, by extrusion or by preferably brazed inserts.
 26. The method of claim 18, wherein the fins are perforated, straight, serrated (with a partial offset), herringbone (zigzagged), and/or louvered.
 27. The method of claim 18, in which the first fluid flows inside at least one tube along a descending vertical direction.
 28. The method of claim 27, in which the first fluid is a fluid that vaporizes in the at least one tube and the second fluid, optionally to be condensed, flows around the fin in a descending vertical direction.
 29. The method of claim 27, in which a fluid to be vaporized (the second fluid) flows around the fin in an ascending vertical direction.
 30. The method for vaporization of at least one liquid derived from air and optionally for condensation of at least one gas derived from air, or which is air, of claim 18,
 31. The method for vaporization of at least one liquid having methane and/or carbon monoxide and/or hydrogen as main component and optionally for condensation of at least one gas having methane and/or carbon monoxide and/or hydrogen as main component of claim
 18. 32. An installation for separating a mixture of fluids by cryogenic distillation in at least one column having at least one heat exchanger operating according to the method of claim
 18. 33. The installation of claim 32, in which at least one of the heat exchangers comprises: a) a reboiler/condenser for vaporization of a liquid by heat exchange with a gas, which condenses inside or outside a distillation column; or b) a subcooler; or c) a regeneration gas heater for a purification unit used for purifying the mixture to be distilled; or d) a dephlegmator; or e) a heat exchanger having passages for cooling the mixture to be distilled at a cryogenic temperature; or f) an exchanger for cooling an interstage of a compressor for compressing the mixture to be distilled or a product of the distillation.
 34. The installation of claim 33, in which a reboiler/condenser located inside the distillation column consists of several exchangers, the exchangers being of different widths so as to fill the entire cross section of the column. 